The present invention relates to a multistage amplifier and integrated circuit. In particular, the present invention is suitable for use for a multistage amplifier that sequentially amplifies signal gains at a plurality of amplification sections in a cascade connection manner, and integrated circuit that is composed of such multistage amplifier or other circuits with large gains.
Normally, a multistage amplifier, where a plurality of amplifiers are constructed in a multistage cascade connection manner and which can obtain high gains, is used in limiter amplifier for an FM radio receiver or Intermediate Frequency (IF) amplifier for an AM radio receiver or the like, so as to obtain larger gains by amplifying input microsignals. Conventionally, ICs where such a multistage amplifier is integrated within a single chip have been also provided.
FIG. 1 is a figure showing a conventional example of an IC chip composed of means of a multistage amplifier. As shown in FIG. 1, the multistage amplifier is composed of n amplifiers 1, 2, 3 (in this example, n=3), which are constructed in a cascade connection manner from an input side to an output side. All amplifiers 1 to 3 are commonly connected to the same power source line 4, and are connected to a power source VDD outside the IC chip 100, via a power source pad 6. Additionally, all amplifiers 1 to 3 are commonly connected to the same ground line 5, and are grounded outside the IC chip 100 via a ground pad 7. All amplifiers 1 to 3 perform amplification operations using the external power source VDD.
In the multistage amplifier structured as mentioned above, a microsignal inputted to a first-stage amplifier 1 is amplified by such amplifier 1 at a predominated level and outputted. The signal amplified and outputted here is inputted to a second-stage amplifier 2, and is further amplified and outputted by such amplifier 2.
In the same manner hereinafter, a third-stage amplifier 3 further amplifies the signal. Herewith, the amplitude of the input microsignal inputted to the first-stage amplifier 1 becomes gradually larger as passes to rear stages, and the signal amplified at the predominated level is finally outputted from the third-stage amplifier 3.
Normally, high gain about 80 dB is required for a limiter amplifier for an FM radio receiver or IF amplifier for an AM radio receiver or the like. An input microsignal on the μV (microvolt) order passes through such multistage amplifier. This enables such signal to be amplified to the mV (millivolt) order.
As shown in FIG. 1, in a conventional multistage amplifier, one power source line 4 is commonly connected to at each stage, and is wired over to the power source pad 6. Thus, differences occur to currents that are sequentially pulled to amplifiers 1 to 3 at each stage from the external power source VDD. At this time, since common impedance occurs on the power source line 4, a potential difference occurs to the power source supplied to the amplifiers 1 to 3 at each stage. Especially, when a circuit with large area such as an amplifier is constructed in a cascade connection manner, the distance from the first-stage amplifier 1 to the power sourced pad 6 becomes long, and a large potential difference occurs due to high impedance. And this potential difference causes a problem as noise.
Additionally, signals are fed back from the rear-stage amplifiers to the early-stage amplifiers through signal feedback loops that are formed by the internal circuit of the amplifiers 1 to 3, the power source line 4 and ground line 5. Therefore, interference between normal amplified signals and feedback signals causes instability of amplification operations, the performance of a multistage amplifier falls. Also, when signals on the mV order at the final-stage amplifier 3 are fed back to the initial-stage amplifier 1, which deals with microsignals on the μV order, oscillations occur if the total loop gain is large.
The present invention is accomplished in order to solve such problems. When a plurality of amplifiers are constructed in a cascade connection manner and the like, in a circuit that has large gains and whose area is large, the purpose of the invention is to prevent noise or oscillations from occurring on the power source line, and to cause such circuit to operate in a stable manner.